1. Field of the Invention
This invention relates to secondary batteries used in, for example, portable electronic equipment. More particularly, it relates to batteries that can take an arbitrary shape, such as a thin shape, and an adhesive used therein.
2. Discussion of the Background
There has been an eager demand for reduction in size and weight of portable electronic equipment, and improvement of battery performance and battery size reduction are indispensable for the realization. To meet the demand, development and improvement of batteries from various aspects have been proceeding. Characteristics required of batteries include high voltage, high energy density, reliability, and freedom of shape design. Of available batteries, lithium ion batteries are secondary batteries that are the most expected to achieve a high voltage and a high energy density and will undergo successive improvements.
A lithium ion secondary battery mainly comprises a positive electrode, a negative electrode, and an ion conducting layer interposed between the electrodes. The lithium ion secondary batteries that have been put to practical use employ a positive plate prepared by applying powder of lithium-cobalt oxide, etc. as an active material to a current collector and a negative plate similarly prepared by applying powder of a carbonaceous material as an active material to a current collector, with a separator, as an ion conducting layer, made of a porous film of polyethylene, polypropylene, etc. and filled with a nonaqueous electrolytic solution being interposed therebetween.
In order to maintain electrical contacts among a positive electrode, a separator and a negative electrode, it is necessary in a conventional lithium ion secondary battery to externally apply pressure by means of a firm battery case made of metal, etc. to maintain all the planar contacts as described in JP-A-8-83608.
JP-A-5-159802 discloses a solid secondary battery, in which an ion conducting solid electrolyte layer and an electrode material layer are heat bonded with a thermoplastic resin binder to make an integral battery body. In this case, since the electrodes and the electrolyte layer are united into an integral body to maintain electrical contacts, the function as a battery is performed without external pressure application. With reference to thin type batteries, those using polymer gel as an ion conductor are known as described in U.S. Pat. No. 5,460,904. The disclosed thin type batteries are characterized in that polyvinylidene fluoride and a hexafluoropropene copolymer are used as polymer gel to join a positive electrode, a separator, and a negative electrode into an integral body.
Conventional batteries being thus constituted, a firm battery case that imposes external pressure to the electrode layer and the electrolyte layer must be used so as to bring them into sufficient electrical contact. As a result, the case which does not participate in electricity generation has a large proportion in the total volume or weight of a battery, which is disadvantageous for production of batteries having a high energy density.
In those batteries in which an electrode layer and a solid electrolyte layer are joined via a binder, the electrode-electrolyte interface is covered with a solid binder, which is disadvantageous from the standpoint of electrical conductivity through the electrode-electrolyte interface as compared with, for example, the above-mentioned type of batteries in which a liquid electrolyte is used and external pressure is applied by means of a battery case. Where a binder is used, on the other hand, no binders equal to a liquid electrolyte in electrical conductivity has been developed as yet, failing to secure equality in electrical conductivity to a battery using a liquid electrolyte.
Further, the thin type batteries using polymer gel fail to have equality in charge and discharge characteristics to those using a liquid electrolyte because there is no gel electrolyte generally known to be equal or superior to a liquid electrolyte in electrical conductivity.
The present invention has been completed in order to solve the above-described problems. Accordingly, an object of the invention is to provide an adhesive with which an electrode layer and an electrolyte layer are joined to establish a satisfactory electrical contact through the electrode-electrolyte layers without using a firm case for external pressure application; and to efficiently produce a thin, light and highly reliable battery with excellent charge and discharge characteristics by using the adhesive.
A first adhesive for batteries according to the present invention is an adhesive for batteries used for adhering an active material layer joined to a current collector to a separator, which comprises a thermoplastic resin, a solvent capable of dissolving the thermoplastic resin, and a neutral and aprotic surface active agent. A solution of the adhesive exhibits improved wetting to bring improved adhesive strength so that deterioration in battery performance can be prevented, and productivity of batteries can be improved. Accordingly, secondary batteries which can take any arbitrary shape, such as a thin shape, secure reliability, and exhibit high charge and discharge efficiency can be produced.
A second adhesive for batteries according to the invention is the above-described first adhesive for batteries, in which the surface-active agent has a polysiloxane skeleton in the molecule thereof. In this aspect the battery performance is further improved, and the workability is improved.
A first battery according to the invention has an electrode laminate composed of a couple of electrodes each having an active material layer bonded to a current collector and a separator sandwiched therebetween and adhered to the active material layer of each electrode with an adhesive comprising a thermoplastic resin, a solvent capable of dissolving the thermoplastic resin, and a neutral and aprotic surface active agent. In this aspect, practical secondary batteries which can take any arbitrary shape, such as a thin shape, secure reliability, and exhibit high charge and discharge efficiency can be obtained.
A second battery according to the invention is the above-described first battery which has a plurality of the electrode laminates. A battery having a multilayer structure and a so increased capacity can be obtained while being compact, securing reliability, and exhibiting high charge and discharge efficiency.
A third battery according to the invention is the above-described second battery, wherein the plurality of electrode laminates are formed by interposing a positive electrode and a negative electrode alternately among a plurality of cut sheets of the separator.
A fourth battery according to the invention is the above-described second battery, wherein the plurality of electrode laminates are formed by interposing a positive electrode and a negative electrode alternately between rolled separators.
A fifth battery according to the invention is the above-described second battery, wherein the plurality of electrode laminates are formed by interposing a positive electrode and a negative electrode alternately between folded separators.